Existing well logging devices can provide useful information about hydraulic properties of formations, such as pressures and fluid flow rates, and can also obtain formation fluid samples, for downhole analysis and/or subsequent uphole analysis. Reference can be made, for example, to U.S. Pat. Nos. 3,859,851, 3,789,575, 3,934,468, and 4,860,581. In a logging device of this general type, known as a formation testing device, a setting arm or setting pistons can be used to controllably urge the body of the logging device against a side of the borehole at a selected depth. The side of the device that is urged against the borehole wall typically includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted into the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon formation pressure can be measured and fluids can be withdrawn from the formation. A formation testing device in widespread commercial use is the “MDT” (trademark of Schlumberger).
Techniques have been developed for determining substances in fluids of a flow line of a formation testing device such as the MDT. In one technique, the fluid is passed through a chamber in the flow line having sapphire windows, a light source, for example an infrared source, is directed at the chamber, and a spectral detector detects the spectrum of transmitted and/or backscattered or reflected light. These and other techniques have been employed to obtain various types of compositional information. Reference can be made, for example, to U.S Pat. Nos. 5,589,430, 5,939,717, 6,465,775, and 6,476,384, and to Brady et al., “Downhole Optical Analysis for Formation Fluids,” Oilfield Review, pp. 21-28, January, 1994. In a form of these devices, the optical density of the NIR radiation as a function of wavelength from about 700 nm to 2500 nm is measured and used to identify fluids based on known absorption peaks of hydrocarbons and water. However, a limitation regarding NIR absorption measurements is that the peaks in the absorption spectra for oil and water are broad and overlapping, which can lead to uncertainty in the differentiation of oil from water. Another device described in the literature employs a Nuclear Magnetic Resonance (NMR) module that is used for monitoring contamination of reservoir fluids by oil base mud filtrate and for identification of reservoir fluids (see M. G. Prammer et al., “The Downhole NMR Fluid Analyzer,” paper N presented at the SPWLA Annual Logging Symposium, Jun. 17-20, 2001). The accurate identification of fluids using NMR is based on measuring diffusion contrasts between the fluids. Diffusion measurements require that the fluids be substantially stationary. Accordingly, such measurements are generally not suitable for identification of fluids as they are flowing in a flowline.
Formation testing devices have also been provided with a resistivity sensor that is used to differentiate conductive water (relatively low resistivity) from hydrocarbons (relatively high resistivity). However, resistivity measurements generally cannot distinguish fresh water hydrocarbons or oil from gas.
It is among the objects of the present invention to provide an improved method and apparatus for downhole determination of properties of sampled borehole fluids, which overcome drawbacks and limitations of prior art approaches.